This invention relates to a photothermographic material.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in medical diagnosis and general photography which can be effectively exposed by means of laser image setters and laser imagers and produce distinct black images having high resolution and sharpness. These thermographic photosensitive materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
On the other hand, the recent rapid progress of semiconductor laser technology has made it possible to reduce the size of medical image output devices. As a matter of course, there were developed techniques relating to infrared-sensitive photothermal silver halide photographic material which can utilize a semiconductor laser as a light source. The spectral sensitization technique is disclosed, for example, in JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. The antihalation technique is disclosed, for example, in JP-A 13295/1995 and U.S. Pat. No. 5,380,635. Since the infrared exposure system permits the visible absorption of sensitizing dyes and antihalation dyes to be considerably reduced, a substantially colorless photosensitive material can be readily produced.
Since spectral sensitizing dyes capable of absorbing infrared radiation, however, generally have a high reducing power due to a high HOMO (highest occupied molecular orbital), they tend to reduce silver ions in photosensitive materials to exacerbate the fog thereof. In particular, these photosensitive materials experience a substantial change of performance during storage under hot humid conditions and long-term storage. If dyes having a low HOMO are used for preventing deterioration of storage stability, spectral sensitization efficiency and sensitivity become low because their LUMO (lowest unoccupied molecular orbital) is relatively low. These problems relating to sensitivity and storage stability arise not only with wet photographic photosensitive materials, but more outstandingly with photothermographic materials.
The reducing power of a dye, of course, tends to increase as its amount becomes larger. A photosensitive material containing a small amount of dye, however, shows short sensitivity because incident light to the photosensitive material is not fully absorbed. Especially in a photothermographic material using a highly lipophilic binder, the adsorption of the dye to a silver halide or photosensitive element is weak, and the sensitivity becomes short unless the dye is added in a sufficient amount.
It is believed that such inconvenience can be mitigated by using a sensitizing dye capable of absorbing light of shorter wavelength. Therefore, there is a need for a photosensitive material suitable for exposure by a red laser rather than an infrared laser. However, in a photothermographic material containing an organic silver salt, the use of a sensitizing dye capable of absorbing red laser light mitigates the problems as compared with the use of infrared dyes, but the occurrence of fog to a practically noticeable extent and performance variations during storage still remain unsolved.